Endocrine cells in the human fetal small intestine.

In this report we describe the time of appearance and ultrastructural features of enteroendocrine (EECs) in the human fetal small intestine (SB) between 9 and 22 weeks gestation. Thirteen distinctive EECs were identified in fetal SB. Two of these, not found in normal adult SB, appeared within the stratified epithelium of the proximal SB at 9--10 weeks. They were arbitrarily termed "primitive" and "precursor" cells. As in all fetal EECs, the pale cytoplasm of the "primitive" cell contains a distinctive population of secretory granules (SGs). Primitive cell SGs average 200-330 nm; some have dense cores with lucent halos while others are filled with a homogeneous dense or flocculent material. The SGs of the "precursor" cells are larger, averaging up to 1 micron in diameter and their contents vary in electron density. A third group of cells not described in normal adult SB was arbitrarily termed "transitional" cells. These have two populations of SGs; one resembles the SGs of the "precursor" cells, and the other resembles the SGs of some of the specific adult type EECs. Transitional EC, S, I and G cells are seen. In addition, mature appearing EC, S, G, I, L, D, and D1 cells were identified by 12 weeks of gestation. The "primitive", "precursor", and "transitional" cells may represent sequential developmental precursors of adult type EECs.